Mass spectrometry (MS) is used widely for identifying and quantifying molecular species in a sample. During analysis, molecules from the sample are ionized to form ions. A detector produces a signal relating to the mass of the molecule and charge carried on the molecule and a mass-to-charge ratio (m/z) for each of the ions is determined.
A chromatographic separation technique may be performed prior to injecting the sample into a mass spectrometer. Chromatography is a technique for separating compounds, such as those held in solution, where the compounds will exhibit different affinity for a separation medium in contact with the solution. As the solution flows through such an immobile medium, the compounds separate from one another. Common chromatographic separation instruments include gas chromatographs (GC) and liquid chromatographs (LC). When coupled to a mass spectrometer, the resulting systems are referred to as GC/MS or LC/MS systems. GC/MS or LC/MS systems are typically on-line systems in which the output of the GC or LC is coupled directly to the MS.
In an LC/MS system, a sample is injected into the liquid chromatograph at a particular time. The liquid chromatograph causes the sample to elute over time resulting in an eluent that exits the liquid chromatograph. The eluent exiting the liquid chromatograph is continuously introduced into the ionization source of the mass spectrometer. As the separation progresses, the composition of the mass spectrum generated by the MS evolves and reflects the changing composition of the eluent.
Typically, at regularly spaced time intervals, a computer-based system samples and records the spectrum. The response (or intensity) of an ion is the height or area of the peak as may be seen in the spectrum. The spectra generated by conventional LC/MS systems may be further analyzed. Mass or mass-to-charge ratio estimates for an ion are derived through examination of a spectrum that contains the ion. Retention time estimates for an ion are derived by examination of a chromatogram that contains the ion.
Two stages of mass analysis (MS/MS also referred to as tandem mass spectrometry) may also be performed. One particular mode of MS/MS is known as product ion scanning (and also as data dependent analysis (DDA)) where parent or precursor ions of a particular m/z value are selected in the first stage of mass analysis by a first mass filter/analyzer. The selected precursor ions are then passed to a collision cell where they are fragmented to produce product or fragment ions. The product or fragment ions are then mass analyzed by a second mass filter/analyzer to obtain a resulting product spectrum. The foregoing process can be repeated for other selected precursor ions of interest.
In connection with mass spectrometry and ionizing a precursor ion to produce characteristic fragments thereof, a collision energy (CE) voltage is selected to impart a desired CE to ions transmitted to the collision cell. The CE may be selected, such as from a lookup table of empirically derived CE values, as a function of the precursor's m/z value or mass and charge state. A collision cell may include a chamber into which an inert gas or a mixture of gases is introduced. The CE is imparted by selecting and applying the CE voltage to induce collisions of the molecules of atoms of the gas of the collision cell.